Ernest Victor Lallier.

An elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer online

. (page 9 of 17)
Online LibraryErnest Victor LallierAn elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer → online text (page 9 of 17)
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gallons per minute at 100 feet piston speed per minute.

To find the horse power of a pump necessary to feed
a boiler evaporating a given number of pounds of water
per hour and carrying a given steam pressure. Mum-
ply the pressure in pounds per square inch, against
which the pump is operating, by the velocity of the
flow of water in feet per minute. Then divide this
product by 33,000 to obtain the required horse power.

To determine the proper size of a pump to feed a
boiler, the first step is to determine the probable amount
of coal burned per hour and the probable evaporation
of the water per pound of coal.

Suppose that each of two boilers has 19 square feet
of grate surface and bums 10 pounds of coal per square
foot of grate surface per hour and each pound of coal
evaporates 8 pounds of water. Then the total evapo-
ration equals 19 X 10 X 8 = 1520 pounds per hour.
1520 -r- 60 = 25.3 pounds of water evaporated per min-
ute for each boiler or 50.6 for both. 50.6 -r- 8| = 6.8
gallons per minute. To provide for all contingencies,
the pump should have at least double this capacity,
or be one that would deliver 13.6 gallons per minute
when working at a moderate speed.



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PUMPS. IS3



INJECTORS.



As it is necessary for tlie cold water supplied by the
pump to be heated on its way to the boiler, this is usually
done by the use of exhaust steam in a feed-water heater,
the exhaust being supplied from the engines operating
the plant. Then, if no engines are running, no steam,
of course, will be available for this purpose. In this
case, the injector may be employed. The principle



Fig. 6i.

upon which the injector works is briefly as follows: A
pipe connection, S, Fig. 6i, to the upper part of the steam
space is made, and the continuation of this pipe into the
injector proper forms a steam nozzle, F.

The end of this nozzle extends almost into the second
nozzle, C, called the combining or suction nozzle; this
connects with, or rather terminates in, a third nozzle or
tube, D, termed the ** forcer." At the end of the com-
bining tube, and before entering the forcer, is an opening



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154 ELEMENTARY STEAM ENGINEERING.

connecting the interior of the nozzle at this point with
the surrounding area.. This area is connected with the
outside air by a check valve, P, opening outward in
. automatic injectors, and termed the overflow valve.

The operation of the injector is based on the fact,
first demonstrated by Giffard, that the motion imparted
by a jet of steam to a surrounding column of water is
sufficient to force it into the boiler from which the steam
was taken and back into the boiler working at a high
pressure. The steam escaping from under pressure
has, in fact, a much higher velocity than water would
have under the same presstire and conditions. The
rate of speed at which steam — taking it at an average
boiler pressure of sixty pounds — travels when dis-
charged into the atmosphere, is about 1700 feet per
second. When discharged with the fuU velocity de-
veloped by the boiler pressure through a pipe, say an
inch in diameter, the steam encounters the water in the
combining chamber. Uniting with the body of water
in the combining tube, it imparts to it a large share of
its speed, and the body of water thus set in motion,
operating against a comparatively small area of boiler
pressure, is able to overcome it and pass into the boiler.
The weight of the water to which steam imparts its
velocity gives it a momentum that is greater in the small
area through which its force is exerted than is the boiler
presstire, although its force has actually been derived
from the boiler pressure itself.

An injector is a very efficient substitute for a feed
pump ; but even though it puts the water into the boiler
at 150° F., this high temperature does not represent a



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PUMPS. IS5

coal saving because the heating is done by steam taken
from the boiler itself, rather than by the waste heat of
the exhaust steam.

Injectors should be used only as supplementary feed
systems, or for portable boilers.

In round numbers it may be stated that for every
10° F. that the feed water is heated before entering the
boilers, i per cent less coal is required to generate the
same horse power. Also, for each io° F. increase in
feed temperature, the boiler capacity is increased i per
cent. By transferring the exhaust of the feed pumps
and other auxiliaries from the main condenser to the
heater, the effective condenser capacity is increased
I per cent for every io° temperature rise of the feed.
By feeding very hot water to the boilers the severe
temperature strains that are the source of leaking tubes
and seams are avoided.

In the injector. Fig. 6i, the steam enters from above,
the flow being regulated by the handle, K. The steam
passes through the tube, S, and expands in the tube, V,
where it meets the water coming from the suction pipe.
The condensation takes place in the tubes, V and C, and
a jet of water is delivered through the forcer tube, D, to
the boiler. Connection passages are made to the cham-
ber surrounding the tubes, C, D, to the chamber, H. If
the presstire in this surrounding chamber becomes
greater than that of the atmosphere, the check valve P
is lifted and the contents are discharged through the
overflow. So long as the pressure in this chamber is
atmospheric the check valve, P, remains closed, and all
the contents must be discharged through the tube, D.



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156 ELEMENTARY STEAM ENGINEERING.

There are three distinct tjrpes of live-steam injectors,
the ^^ simple fixed nozzle," the ^'adjustable nozzle,"
and the '^ double nozzle."

The first has one steam and one water nozzle which
are fixed in position but are so proportioned as to yield
a good result. There is a steam pressure for every
instrument of this type at which it will give a maximum
delivery, greater than the maximum delivery for any
other steam pressure either higher or lower.

The second type has but one set of nozzles, but they
can be so adjusted relative to each other as to produce
the best results throughout a long range of action; that
is to say, it so adjusts itself that its maximum delivery
continually increases with the increase of steam pressure.

FEED- WATER HEATERS.

Feed-water heaters are made in two general forms,
known as open and closed heaters. They resemble, in
principle, jet or surface condensers in so far that, in
the one case, there is a mixture of cooling and condensed
water; in the other, the water is heated by flowing over
heated surfaces. One disadvantage of the open heater
is that oil and grease from the exhaust steam become
mixed with the feed water.

Enclosed heaters are such as shown in Fig. 62, where
a metallic vessel of suitable size contains a number of
tubes or coils connected to partitions, so placed in the
ends of the main vessel that the entering feed water is
caused to flow or circulate through them in its passage
from one end of the vessel to the other.

In the illustrationi the exhaust steam is allowed to



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PUMPS. IS7



Fig. 62.



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158 ELEMENTARY STEAM ENGINEERING.

enter at a and fill the larger vessel, coming in contact
with the surfaces of the pipes in such a manner as to
thoroughly heat them and then passes out at h. This
heat is absorbed by the circulating feed water which
passes from the inlet c through the pipes, as shown
by the arrows, and out at d to the boiler in a heated
condition. The circulating pipes should preferably be
made of brass or copper, and the points where they
pierce the inner heads or plates should be carefully
constructed and packed in order to remain perfectly
tight and yet allow for expansion and contraction due
to change in temperature. At e is a settling chamber
to collect such solid material as may be liberated from
the water. On both sides of each piece of apparatus, as
feed-water heaters, pumps, injectors, etc., valves should
be placed in order that the apparatus may be cut out
of the line if necessary. On the feed line, a valve should
be placed as close to the boiler as possible and just
before this valve, usually of the ordinary globe type, a
check valve must be used in order to prevent the pres-
sure in the boiler from forcing the water out on the line «-
again.

QUESTIONS.

1. What is a steam pump?

2. V^at forces water up into the suction pipe?

3. Which cylinder is the largest in steam ptmips and why?

4. How much does a cubic foot of water weigh?

5. How many gallons are there in a cubic foot?

6. How much does a gallon of water weigh?

7. How many cubic inches in a XT. S. Standard gallon?

8. Given the height of a gallon of water, how would you calcu-
late its pressure per square inch?



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PUMPS. IS9

9. How would you detennine the height when the pressure is
given?

10. How is the total resistance per square inch overcome by a
water piston determined?

11. What difference does it make whether the supply is above or
below the pump?

12. How would you calculate the pressure against which a
pump will work?

13. How would you calctilate the steam pressure necessary to
operate a boiler feed ptunp?

14. How would you calculate the diameter of the water piston
for a pump?

15. How would you calculate the diameter of the steam piston
for a ptunp?

16. What is the limit of speed for a pump?

17. How would you calctilate the capacity of a pump for 100 feet
piston speed per minute?

18. How would you calctilate the diameter of a pump piston to
move a given quantity of water per minute?

19. How is tiie size of pipe, necessary to deliver a certain quan-
tity of water, calculated?

20. What is the difference between the simple or single steam
pump and the duplex pump?

21. How would you set the steam valves of a duplex pump?

22. When and why are steam valves given ** lost motions "?

23. How many ports has a common duplex pump?

24. How many water valves has a duplex pump?

25. How high will a pump lift water?

26. Against what presstire will a pump work?

27. What can you say in regard to the pumping of hot water?

28. What is the purpose of an air chamber on a pump?

29. What is the effect of air in the suction pipe?

30. How would you find the horse power necessary to elevate
water to a given height?

31. If a stand pipe is 100 feet high, what pressure will the gage
show at the bottom?

32. What is a vacuum?

.33. How many gallons per hotu: would be pumped by a pump
having a plunger 6 inches in diameter, stroke 24 inches, speed 100
ft. per minute?



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l6o ELEMENTARY STEAM ENGINEERING.

34* How many foot pounds work will be done if the water is
pumped into a tank 50 feet above the supply?

35. How would you determine the efficienqr of a pump?

36. What is an injector?

37. Why should both an injector and a pump be provided?

38. Describe a steam pump.

39. Describe a feed-water heater.



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CHAPTER XVI.
CORLISS ENGINE.

One of the greatest advance steps in engineering
work was the invention, by George Corliss, of the type of
engine which bears his name.

The cylinder of the Corliss engine has four valves
placed, as one might say, at the comers of the cylinder
section. The two upper valves are the steam valves,
the two lower are the exhaust valves.



Fig. 63.

The seats on which they operate consist of openings
bored through the cylinder casting at right angles to its
length. The valves themselves are in the form of

161



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l62 ELEMENTARY STEAM ENGINEERING.

cylinders, carefully fitted to these openings and having
a portion of the central section removed as shown in
Fig. 63. By reference to the figure it will be seen that
steam entering through the main steam pipe s passes
through a chamber close to the cylinder casting, thus
helping to maintain the temperature of the cylinder,
and enters the cylinder through one of the steam valves
V or v'. After doing its work it passes out through
one of the lower valves e or e', passing through another
chamber to the exhaust pipe «. The valves might be
called oscillating valves, as they do not produce a com-
plete revolution but swing on their centers to open and
close the port openings. The ports, it will be noticed,
are large, allowing rapid admission of the steam, and
the valve being so near the cylinder proper there is very
little length of port. Consequently, there is little energy
lost due to a quantity of steam remaining in the ports,
which enables close regulation. Referring to Fig. 64,
illustrating the exterior view of the Corliss engine, it
shows the reach rod r, which is operated by the eccentric
through the eccentric rod and rocker arm. The recipro-
cating movement of this rod is employed to produce
an oscillating movement of the wrist plate w^ mounted
on a stud at the side of the cylinder. This movement
of the wrist plate produces a corresponding motion of
the four studs bb-b'b' placed upon its face. This move-
ment in turn is imparted by means of the adjustable
rods cccc to the four valves. The lower studs 6'6' are
directly connected to the exhaust valves by means of the
rods cc and the cranks dd keyed to the valve stems.
The two upper studs operate the steam valves through



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CORLISS ENGINE.



163




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i64 ELEMENTARY STEAM ENGINEERING.

the valve mechanism or release motion as follows, see
Fig. 65,

In admitting the steam the wrist-plate rod moves to
the right and pulls with it the bell crank c\ which has on
one of its ends the hook e. In moving up, this hook
engages with a pawl h fastened to the valve stem a,
and lifts this pawl, rotating the valve on its center and
allowing the admission of steam. The hook is pivoted
as shown at d and carries a roller r fastened to a short
arm on the reverse side of the supporting bell crank.
The governor rod connected at 2 extends to a suitable
crank on the engine governor and as the engine increases
or decreases in speed the governor causes this rod to
move J to the right or to the left. In doing so it turns
the crank J upon its center causing the little roller on the
opposite arm of the crank j' to vary its position on the
circumference of a circle, having a as a center. At some
point in the movement of the valve the roller controlling
the hook e will come in contact with the roller on the
bell crank f connected to the governor rod. When
this occurs the hook e is forced out, thus releasing the
pawl on the valve stem.

The point at which these two rollers come in contact
being regulated by the governor, the release of the valve
occurs at an earlier or later period of the stroke as may
be required by the work then being done. On the valve
being released in this manner, it is free to close. In
order to enable rapid closing there is pivoted to the pawl
a vertical rod extending downward to the dash-pot.
This may be placed in either a vertical or an inclined
position, although the principle is the same in all. One



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CORLISS ENGINB.



I6S




Fig. 65.



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i66 ELEMENTARY STEAM ENGINEERING.

form is shown in Fig. 66 which consists of a stationary
base a which is fixed to the engine frame, and a movable

plunger h connected by
the vertical rod r to the
valve pawl. The plunger
is fitted with packing
rings, in order to produce
an air-tight joint. When
the hook is opening the
valve, the rod is lifted,
drawing the plunger up-
wards. An annular space
is thus left below the
plunger in which a partial
vacuum is formed.
As soon as the valve
t has been released by the
hook the atmospheric
pressure on top of the
* piston pushes it rapidly

downward, due to the partial vacutmi below it. By
means of the pull thus exerted on the rod it closes the
steam valve almost instantaneously. The very small
amount of air remaining in the dash-pot cylinder serves
to cushion the plunger and prevent it from striking the
bottom of the cylinder. Should too much air accumulate
in here it may be allowed to escape by means of the
check valve e.



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CORLISS ENGINE. 167

COMPOUND ENGINES.

As we have already seen, economy is obtained by
the use of high-pressure steam used expansively. On
this principle it would be to otir further advantage to
increase our boiler pressure to 150 pounds or more per
square inch, and produce a sufficient number of expan-
sions to bring the back pressure considerably below that
of the atmosphere. This would involve a large number
of expansions, and consequently an extremely long
engine cylinder. The difficulty in the way of employing
such a cylinder is, in the first place, in the machining.
Such a cylinder would be greatly distorted, largely due
to the fact that, in the period of time elapsing while the
piston was traveling from one end of the cylinder to
the other, and due to the increased volume with conse-
quently lowered pressure, the variation in temperature
would be so great as to produce considerable condensa-
tion of steam. In order, then, to obtain good results
without consequent ill effects, compound engines are
used. These consist briefly of engines having two or
more cylinders, acting on and delivering their power to
the same crank shaft.

If, for example, steam at 150 pounds per square inch,
occupying a voltmie of 2.96 cubic feet, were admitted
to an engine and were allowed to perform its work and
expand until it had reached the pressure of 75 pounds
per square inch, this work would have been done under
favorable conditions; but instead of allowing steam to
exhaust from this cylinder at a pressure still sufficient
to do good work and to escape to the atmosphere and
thereby be lost, in the compound engine we now send



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i68 :el£M£ntary steam engineering.

it into the cylinder of a second engine, so that the ter-
minal pressure of steam of the first engine becomes the
initial pressure steam of the second engine.

On account of its reduced pressure its volume would
have increased to 5.68 cubic feet; therefore, the cylinder
of the second engine should be proportionally larger in
volume, so that at each stroke it might accommodate
the steam exhausted from the first engine during each
stroke. If, now, the connecting rod of both of these
engines is connected to the same shaft, we will have the
advantage of the high-pressure steam, the great num-
ber of expansions and the combination of the total power
of the engines delivered to one shaft. This, in brief,
is the compound engine. It may consist of two or more
cylinders known respectively as double, triple, quad-
ruple expansion, etc. The cylinder getting the steam
first is called the high-pressure cylinder. That receiv-
ing it last is called the low-pressure cylinder. Where
three or more cylinders are employed the others are
called first and second intermediates, as the case may
be. Compound engines are also designated according
to their construction. When the cylinders are placed
one behind the other, the piston being on the same
piston-rod and having but one connecting rod and crank,
it is called a tandem compound. Fig. 67, the path of the
steam being indicated by the arrows.

If the engines are placed side by side having separate
connecting rods and cranks they are called cross com-
pounds. Fig. 68. While the latter type occupies con-
siderable space it has the advantage in the fact that the
cranks may be placed at right angles to each other.



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CORLISS ENGHVE.



169



thereby insuring ease in starting, for while one engine
may be on the dead center the other will be at its point
of mayimtim turning power. Also, if desired, one engine




Fig. 67.



Fig. 68.



may readily be disconnected from the shaft and the other
one operated by itself.

These engines are sometimes built vertically and
sometimes a combination of both the vertical and the
horizontal type is employed in the same engine. When
the steam passes from the high to the low pressure



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170 ELEMENTARY STEAM ENGINEERING.

cylinder it is occasionally led to a receiver connected
between two engines in which it may be slightly super-
heated before entering the second cylinder. This id
done in order to get rid of any moisture which may
have collected Fig. 68, a.

In all cases the length of stroke of the pistons are
alike; therefore, the increase in volume is gained by
increasing the diameter of the lower pressure cylinders.
In considering the use of high-pressure steam in con-
nection with compotmd engines, it is necessary to de-
termine how many expansions we desire to obtain. If,
for example, the steam pressure be limited to 185 potmds
absolute and the terminal pressu):e be considered as
6 potmds absolute, we apply the following rule:

Divide the absolute iniUal steam pressure by the
absolute terminal pressure and the quotient will he

i8s
the total number of expansions* Thus we have -7-

which equals 30.8 expansions. As there will be a slight
loss between the cylinders thi3 wUl produce actually
about 30 expansions.

Having determined the total number of expansions,
in order to find the approximate number in each cylinder
we employ the following rule:

For double expansion, extract the square root of
the total. For triple expansion^ extract the cube root
of the total. For quadruple expansion extract the
fourth root of the total. Therefore in a triple expansion
engine having 30 expansions the approximate number
obtained in each cylinder would be \^ = 3'i*

It is customary and preferable in actual operation



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CORLISS ENGINE. 171

to make the cut-off in the high-pressure cylinder a little
earlier than in the others. This will ordinarily produce
the following results:

high pressure 3.2 expansions

intermediate 3.1 **

low pressure 3.0 "



Total
30



In the case of compotmd engines the condenser is of
special value, enabling us to obtain the greatest number
of expansions by bringing the ter-
minal pressure below that of the
atmosphere.

CONDENSERS.

There are many styles of con-
densing apparatus, but they may be
placed as belonging to two types
known as first, jet condensing, and
second, surface condensing appa-
ratus. In either case quantities of
water are required to produce the
cooling action necessary; therefore,
particularly in cities, unless large
quantities of water can be obtained
from artesian wells or similar sources,
condensing is not resorted to unless
in a plant of fairly large size which
will run economically even with the
extra expense of the water and inci-
dental apparatus. The jet condensing Fig. 69.
apparatus, Fig. 69, consists of a vertical hollow vessel &,



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172



ELEMENTARY STEAM ENGINEERING.



one end connecting with the cylinder by the exhaust
pipe e and the lower end with the air pump at c, the
whole system being perfectly air-tight Into the con-
denser and near the inlet for exhaust steam is intro-
duced a spray pipe or sprinkler a by means of which
a shower of cold water entering at i is allowed to mingle
with the exhaust steam, condensing it and producing a



m



^ssss



a:



r



sssss



^ *>/jv//j^j^^jj?jjjjj^^j/j^m



V//???^?J/^>^/??>J?/J?J?»/A



3



J



Fig. 70.



^ss:s



s ujj??j???^?^?jj^?^n??jjjjn ^^^^^^^^/^////^m^^^^^/^/^



^



mixture of injection water and condensed steam which
falls to the lower portion of the condenser and is pumped
out of it to the hot well. The surface condenser does
not mix the water and exhaust steam. A series of
tubes are placed in the main body of the condenser.



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CORLISS ENGINE. 173

through which a quantity of cold water is maintained in
circulation. The exhaust steam entering the body of
the condenser, surrotmding and coming into contact
with the cool surfaces of the pipes containing the circu-
lation water, is condensed and disposed of as in the
previous case.

In Fig. 70 is shown a surface condenser. The cooling
water entering at a passes through the tubes as shown
by the arrows, and out at b. The exhaust steam enters
at c, the water of condensation being pumped out at e.

QUESTIONS.

1. Describe briefly a Corliss engine.

2. Wherein does it di£fer from the ordinary slide-valve engine?

3. Describe the valve motion?

4. What is the object of the dash-pots?

5. What is a compound engine?


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Online LibraryErnest Victor LallierAn elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer → online text (page 9 of 17)